The invention relates to a method and an apparatus for the removal of solder.
Electronic components which are soldered at junctions on a circuit board today form the basis of any kind of electronic apparatus. On a circuit board of this kind, circuits are especially used as electronic components which are known by the names, xe2x80x9cball grid array,xe2x80x9d xe2x80x9cflip chipxe2x80x9d and xe2x80x9cchip scale package.xe2x80x9d
In the event of malfunction the electronic components must be replaced. For this purpose they are desoldered from the circuit board. This signifies that the solder at the junctions is melted and the electronic components are removed from the board while the solder is still fluid.
After the electronic components are removed, the residual solder remaining on the circuit board must often be removed. A reason for this is, for example, the content of components that are dissolved in the solder, making impossible to the reuse the solder.
A simple method for the removal of residual solder is to remove it by means of a piece of fine stranded tinned copper wire. The residual solder is first melted and then is absorbed by the stranded wire by virtue of a capillary effect of the stranded wire. The stranded wire is pressed lightly against the circuit board for this purpose.
To enable the residual solder to be absorbed by the stranded wire the use of a flux is necessary. After the components are soldered back in, the flux may interfere with the adhesion of an xe2x80x9cunderfiller.xe2x80x9d This term refers to a plastic which is applied in liquid form between the component and the circuit board, where it is cured. An underfiller relieves mechanical stress on the junctions. Furthermore, the stranded wire method of the removal of residual solder from very large surfaces is tedious and difficult. Also, there is the danger that, when the stranded wire is lifted from the circuit board the solder residue may solidify again, so that as the stranded wire is further raised, conductors printed on the circuit board may become damaged.
US-A-5,284,286 discloses a porous copper block with which the residual solder is removed from a circuit board, also by means of capillary action. The copper block has projections which absorb the residual solder. Small amounts of solder which adhere very strongly to the junctions of the circuit board are not, however, absorbed by the known copper block. Furthermore the use of flux is necessary in this known process.
Furthermore known is a vacuum desoldering bulb in the tip of which a hole is provided such that the solder fused to the tip can be sucked away through the hole. It is a disadvantage in this case that this vacuum desoldering bulb has to be guided by hand over the circuit board. Thus damage is often done to the fine conductor strips. Furthermore, small amounts of solder cannot be removed with the known vacuum desoldering bulb, since the suction power of the vacuum bulb does not suffice to remove these small amounts of solder adhering to the circuit board.
The present invention is therefore addressed to the problem of offering a method and an apparatus by which the solderxe2x80x94especially residual solderxe2x80x94can be completely removed from a circuit board, by which damage to the conductor strips is avoided, and by which even a large area on the circuit board can be quickly and easily freed of the solder.
This problem is solved according to the invention by a method with the features of claim 1 as well as an apparatus with the features of claim 9.
The method of the invention is based on the knowledge that, after melting, the residual solder is shifted by a pushing means from soldering points of a circuit board, first into other areas of the circuit board which have far less tendency to adhere to the residual solder. From these areas the residual solder is very easy to remove, so that even very small amounts of solder which due to their surface tension are not driven by a gas stream are removed from the circuit board.
Alternatively, a plunger element is used which is placed on the junctions with the solder to be removed and displaces the solder from the junctions.
After the aforesaid shifting or displacement of the residual solder, the residual solder is preferably removed from the circuit board by aspiration. Advantageous in this case is the use of a vacuum aspiration device.
In an advantageous embodiment of the method of the invention, the molten residual solder is shifted to areas of the circuit board which are not wettable by the solder due to their coating. The residual solder cannot solidify in these areas, so that it is very easily sucked away from them.
The at least one pushing means is preferably pressed against the circuit board with an adjustable force. In this embodiment of the invention it is advantageous that the pushing means is always pressed against the circuit board with such a sufficient force that even fused solder adhering very strongly to the junctions and to the circuit board is moved by the pushing means.
In a preferred embodiment of the method of the invention the fusion of the residual solder is performed by a hot gas. Suitable especially for this purpose is an inert gas, nitrogen for example. The use of an inert gas has the advantage that it does not react chemically either with the surface of the circuit board or with the residual solder. The formation of new substances which might impede the removal of the solder from the circuit board is thus excluded.
In another embodiment, before the gas is fed to the circuit board, formic acid is added to it in order to reduce any oxide coatings present on the melted solder. In this manner the residual solder can then be better removed from the circuit board.
In another embodiment of the method of the invention, the residual solder is aspirated at a faster rate than the rate of delivery of the hot gas. In this manner a gas flow is formed which primarily heats the amount of the residual solder that is to be melted, without heating areas of the circuit board adjoining the area in which the component was disposed on the circuit board. In this manner it is also assured that the molten and already displaced solder will not unintentionally be distributed back onto the circuit board, but will be aspirated immediately after it shifts. It has been found especially that small amounts of solder are driven by the suction flow and aspirated just when they are moved by the vacuum flow.
An apparatus according to the invention, with the features of claim 9, is especially suited for the practice of the method of the invention described above.
The apparatus according to the invention is based, like the method of the invention, on the knowledge that the residual solder can be removed very easily from a circuit board if after it is melted it is shifted by means of at least one pushing means from junctions on the circuit board, at first into other areas of the circuit board which offer far less adhesion for the residual solder. From these areas the residual solder, especially small amounts thereof, are very easy to remove.
Alternatively, a plunger element is provided to displace the molten solder from the junctions.
Preferred embodiments of the apparatus of the invention are described below. As regards their advantages, see the statements set forth above insofar as they are not explicitly mentioned hereinafter.
The apparatus according to the invention has preferably a gas feeder by which hot gas is delivered to the circuit board to melt the residual solder. The gas feeder is advantageously made with such thin walls that it has a very low heat capacity. Thus very little heat passes from the hot gas to the gas feeder, so that the hot gas strikes the circuit board at virtually the same temperature at which it is delivered into the gas feeder. A reliable and complete melting of the residual solder is thus assured.
In a special embodiment of the apparatus of the invention the outer circumference of the gas feeder is configured such that it covers the entire circuit board area from which the solder is to be removed. In this manner all of the solder to be removed is melted in a single step.
The pushing means is preferably disposed within the outer circumference of the gas feeder. This assures that even while the residual solder is being shifted from the junctions of the circuit board the residual solder will not solidify.
In an additional embodiment of the apparatus of the invention the gas feeder completely surrounds the pushing means, so that when the gas feeder is placed on the circuit board the contact area between the gas feeder and the circuit board is sealed off. In that case the gas feeder is disposed on the pushing means such that it does not change its position on the circuit board when the pushing means moves. The arrangement of the gas feeder on the pushing means is accomplished preferably by means of a resilient connection. The sealed configuration and the described arrangement of the gas feeder on the pushing means has the advantage that only a certain area of the circuit board is heated. Junctions in other areas of the circuit board, at which additional electronic components are soldered, are not heated, so that the contacts at these junctions are not accidentally broken.
The pushing means is preferably made of a heat-resistant and resilient material. Especially suitable for this is silicone rubber. A pushing means of this material is not deformed by the hot gas, so that a sufficient pushing effect is always made available. The resiliency also assures that the pushing means is always pressed against the circuit board only with such light pressure that the circuit board, especially its conductor strips, will not be damaged.
In a preferred embodiment the apparatus of the invention has a suction device, especially a vacuum aspirating device, by which the molten solder is sucked up from the circuit board.
The aspirating device is preferably disposed at least partially on the pushing means. A central arrangement of the suction device on the pushing means is advantageous. At the same time provision is made such that the pushing means forms at least partially the outer wall of a suction passage.
The pushing means is preferably arranged between a placing means and a suction device such that the part of the pushing means pushing the molten solder is pressed forward by an adjustable force between the placing means and the suction device.
In a further embodiment of the apparatus of the invention the suction device has at least one opening of the suction passage whose outside dimension corresponds preferably at least partially to the outside dimension of the pushing means. This assures that all of the residual solder that is pushed by the pushing means is sucked up from the circuit board.
The pushing means and/or the opening of the suction device are preferably of rectangular configuration. Alternatively, or in addition thereto, the pushing means has at least one rectangular or triangular groove. Furthermore, the pushing means has as pushing surface a substantially rectangular element whose length corresponds to the length of a row of soldering points on the circuit board.
A plurality of grooves of the pushing means are preferably arranged at a distance from one another that corresponds to the spacing of the soldering points of the circuit board. It is advantageous in this case that one groove can be disposed over one soldering point or over soldering points arranged in a row. After the residual solder is melted it is pushed by means of the lateral surfaces of the grooves from the junctions onto the circuit board. With this arrangement it is possible by means of a slight movement of the pushing means to remove from the circuit board all of the residual solder situated in the junctions or the adjacent areas of the circuit board.
In a further embodiment of the invention the plunger element has rows of grooves and ridges, the ridges corresponding to rows of junctions on the circuit board, and when the plunger element is placed on the circuit board they are placed on the row of soldering points in order to displace the solder.
In an additional preferred embodiment of the apparatus according to the invention, means are provided which hold the apparatus at a predetermined distance above the surface of the circuit board and/or press the pushing means against the circuit board with an adjustable force.
These means assure that only those parts of the apparatus touch the circuit board whose contact therewith is really necessary for the removal of the residual solder, especially those of the pushing means. This contact takes place also with only a specific force in order not to damage the circuit board. Holders and placing means already known are suitable for this purpose.
Furthermore, the apparatus according to the invention preferably has means whereby the apparatus can be displaced relative to the position of the circuit board and parallel to its surface. Suitable for this purpose are especially micrometer screws or stepper motors. The position of the apparatus with respect to the surface of the circuit board can thus be precisely established.
Embodiments represented in the drawing are intended to explain ideas on which the invention is based.